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Thick Film Heaters Go Anywhere
Some of the shapes of ceramic substrates used at Res-Net Microwave, Inc.

It's a well known fact that passing current through a thick film resistor can transform it into a heating element. Such a phenomenon is a nuisance in thick film hybrid circuits where resistors need to operate at low temperature without long-term degradation to their electrical properties. However such an occurrence can be used to great advantage in generating heat in a controllable manner. Thick film heaters have come a long way and find application in many diverse fields with cost and space advantages.

Heaters of this type have applications in domestic food and water heaters; gas sensors/detectors; semiconductor wafer heating chucks and process equipment; medical and biological equipment.

Thick film technology is readily available as thick film pastes (inks). It allows direct deposition on substrates to be heated; has been well established for custom designs. The technology can be used to design control and sensing circuits on the same substrate as the heater. Preferred heater substrate materials readily available include Alumina (Al2O3) and Aluminum Nitride (AlN) ceramics, and these can be used in many applications.

In designing thick film heating elements, layout and geometry are very important in maintaining extremely uniform heat distribution on the surface of the substrates, without creating "hot spots". By varying the density of the heating elements in the design, the heat output across the surface can be varied and controlled.

Using Both Sides
Thick film heaters can be fabricated on one or both sides of the ceramic substrate. Thick film conductor and resistor pastes are used for heating elements, with high temperature glass ceramic as an overglaze and conventional UV cured polymer paste as a protective solder mask. Due to the planer profile of the substrate, surface mount components, including on-board sensors can provide independent circuitry to control and maintain the desired temperature. Most important physical properties of the substrate materials for heaters are thermal conductivity and temperature coefficient of thermal expansion (CTE) or thermal shock resistance. The two best performing ceramic substrates for heaters as previously stated are Alumina and Aluminum Nitride.

Thick film heaters provide several advantages. They are compact and provide for a low profile design; they can be designed to provide uniform heat distribution; they are capable of very fast ramp up and down rates (<100°C/sec.).

Thick film heaters ensure direct surface contact for optimum heat transfer precisely where it is needed. Due to the low profile and compact design, thick film heaters on ceramics can replace existing heaters enabling improvements in product design and saving overall cost and space. Ceramics for thick film heaters can be made into virtually any geometrical shape that an application demands or where conventional heaters can not be used. Substrate shapes can be produced by either pressing ceramic powder in a mold at a high temperature or by using a high power laser to cut the form. Res-Net Microwave, Inc. has the capability to custom design and manufacture thick film heaters on alumina and aluminum nitride to meet customer specifications. The company's success in producing high quality heaters for many applications is based on the fact that it can draw upon the years of experience in design and manufacturing of thick film passive microwave and resistive products and capabilities available within its sister companies.

Contact: Res-Net Microwave, Inc., P.O. Box 18802, Clearwater, FL 33762 727-530-9555 fax: 727-535-3508 E-mail: res_sales_service@electrotechnik.com Web:
http://www.res-netmicrowave.com

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